This application claims the benefit of Korean Patent Application Nos. 1999-552862 filed on Nov. 26, 1999 under 35 U.S.C. xc2xa7119, the entirety of each of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed therebetween. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as shown in FIG. 1, which shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. However, this has a problem that the battery can not be used for a long time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2 shows a sub-pixel of a typical reflective LCD device 100 in plane. A plurality of gate lines, including (Nxe2x88x921)th gate line 6 and Nth gate line 8, are spaced apart from each other, and a plurality of data lines, including Nth data line 2 and (N+1)th gate line 4, are arranged perpendicular to the gate lines. In an area defined by the gate and date lines, a reflective electrode 10 is positioned. The gate and data lines and the reflective electrodes make a shape of an array matrix.
In the Nth gate line 8, near a cross point of the Nth gate and data lines 8 and 2, a gate electrode 18 is positioned, and a source electrode 12 is positioned in the Nth data line 2. The source electrode 12 overlaps a portion of the gate electrode 18. Spaced apart from the source electrode 12, a drain electrode 14 is positioned and overlaps a portion of the gate electrode 18. The drain electrode 14 electrically contacts a reflective electrode 10 via a drain contact hole 16 that is formed on the drain electrode 14. Conventionally, the reflective electrode 10 is a metal that has a superior reflexibility.
With reference to FIG. 3, a cross-sectional structure of the conventional reflective TFT-LCD device shown in FIG. 2 is described in detail.
On a substrate 1, the gate electrode 18 and the gate insulating layer 20 are positioned sequentially. The gate insulating layer 20 covers the gate electrode 18. On the gate insulating layer 20, a semiconductor layer 22 is positioned, and the source and drain electrodes 12 and 14 that contact the semiconductor layer 22 are positioned.
A passivation layer 24 is positioned over the overall surface of the substrate 1. On the passivation layer 24, the drain contact hole 16 is positioned to expose a portion of the drain electrode 14. The reflective electrode 10 is positioned on the passivation layer 24 and contacts the drain electrode 14 via the drain contact hole 16.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed.
FIG. 4 shows a conventional transflective LCD device. As shown in FIG. 4, the transflective LCD device includes gate line 50 arranged in a transverse direction, data line 60 arranged in a longitudinal direction perpendicular to the gate line 50, a thin film transistor xe2x80x9cTxe2x80x9d (TFT) located near the cross points of the gate and data line 50 and 60. Each of the TFTs xe2x80x9cTxe2x80x9d includes gate, source, and drain electrodes 52, 62, and 64. The gate and source electrodes 52 and 62 are extended from the gate and data line 50 and 60, respectively. The transflective LCD device further includes a reflective electrode 68 and a pixel electrode 70. The pixel electrode 70 is electrically connected with the drain electrode 64 via a first contact hole 66, and the reflective electrode 68 is electrically connected with the pixel electrode 70 via a second contact hole 67. The reflective electrode 68 is made of an opaque conductive material and preferably the same material as the gate electrode 52, and the pixel electrode 70 is made of a transparent conductive material such as indium tin oxide (ITO). The reflective electrode 68 has a light transmitting hole 72 for transmitting light from a backlight device (see 102 in FIG. 5). The light transmitting hole 72 may have a circular or a rectangular shape and thus is not limited in its shape. The pixel electrode 70 should have a sufficient area to cover the light transmitting hole 72.
As shown in FIG. 5, the conventional transflective LCD device includes upper and lower substrates 106 and 108 with a liquid crystal layer 100 interposed therebetween. The upper substrate 106 includes a color filter 104, and the lower substrate 108 includes a switching element (not shown), a pixel electrode 70 and a reflective electrode 68. A protection film 74 is interposed between the pixel and the reflective electrodes 70 and 68. The reflective electrode 68 is made of an opaque conductive material having a good reflectance, and a light transmitting hole 72 is formed therein. The transflective LCD device further includes a backlight device 102. The light transmitting hole 72 serves to transmit light 114 from the backlight device 102.
The transflective LCD device in FIG. 5 is operable in transmissive and reflective modes. First, in reflective mode, the incident light 110 from the upper substrate 106 is reflected on the reflective electrode 68 and directed toward the upper substrate 106. At this time, when electrical signals are applied to the reflective electrode 68 by the switching element (not shown), the phase of the liquid crystal layer 100 varies and thus the reflected light 120 is colored by the color filter 104 and displayed in the form of images.
Further, in transmissive mode, light 114 generated from the backlight device 102 passes through portions of the pixel electrode 70 corresponding to the transmitting hole 72. When the electrical signals are applied to the pixel electrode 70 by the switching element (not shown), phase of the liquid crystal layer 114 varies. Thus, the light 114 passing through the liquid crystal layer 100 is colored by the color filter 104 such that images are displayed.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power. However, since the reflective electrode has a the transmitting hole 72, the conventional transflective LCD device has a very low light utilizing efficiency compared to either the reflective LCD device or the transmissive LCD device alone.
In the reflective mode of the transflective LCD device, incident light enters the color filter 104 and is reflected on the reflective electrode 68 and reenters the color filter 104. That is, the light passes through the color filter twice. But, in the transmissive mode, light from the backlight 102 passes through the color filter only one time. Thus, the color purity that users perceive varies according to the mode of the LCD device.
As shown in FIG. 6, the conventional transflective LCD device includes a plurality of pixels, and a pixel 200 includes three different sub-pixels of R(red), G(green), and B(blue). Each sub-pixel R, G or B has the same configuration shown in FIGS. 4 and 5.
In an actual transflective LCD device according to the concept of the conventional transflective LCD device described above, since one sub-pixel has both of the transmissive and reflective portions, various masks should be adapted with respect to structures and ratios of the reflective and transmissive portions. Further, a protection film should be interposed between the reflective and the pixel electrodes, which should contact electrically with each other. Accordingly, the fabricating process and cost becomes complicated and high.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention is to provide a transflective LCD device that can be fabricated via simpler processes.
Another object of the invention is to provide a transflective LCD device that can achieve substantially accurate reflection versus transmission ratios.
In accordance with the purpose of the invention, as embodied and broadly described, the invention includes a transflective liquid crystal display device, including: first and second substrates opposing each other; liquid crystal material interposed between the first and second substrates; first and second electrodes, arranged in correspondence to the first and second substrates, respectively, to apply an electric field to the liquid crystal material; reflective pixel electrodes being positioned between the liquid crystal material and the second substrate; transparent pixel electrodes being positioned between the liquid crystal material and the second substrate; a color filter layer positioned between the first substrate and the liquid crystal material, the color filter layer having first portions aligned with the reflective pixel electrodes and second portions aligned with the transparent pixel electrodes; and a backlight device under the second substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.